首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   870篇
  免费   1篇
公路运输   223篇
综合类   35篇
水路运输   326篇
铁路运输   32篇
综合运输   255篇
  2022年   7篇
  2021年   4篇
  2020年   9篇
  2019年   10篇
  2018年   12篇
  2017年   21篇
  2016年   21篇
  2015年   15篇
  2014年   16篇
  2013年   133篇
  2012年   32篇
  2011年   39篇
  2010年   29篇
  2009年   35篇
  2008年   38篇
  2007年   27篇
  2006年   16篇
  2005年   19篇
  2004年   16篇
  2003年   21篇
  2002年   8篇
  2001年   11篇
  2000年   20篇
  1999年   9篇
  1998年   13篇
  1997年   17篇
  1996年   21篇
  1995年   25篇
  1994年   5篇
  1993年   13篇
  1992年   17篇
  1991年   8篇
  1990年   13篇
  1989年   5篇
  1988年   14篇
  1987年   10篇
  1986年   10篇
  1985年   12篇
  1984年   6篇
  1983年   15篇
  1982年   12篇
  1981年   18篇
  1980年   9篇
  1979年   9篇
  1978年   9篇
  1977年   6篇
  1976年   6篇
  1975年   13篇
  1974年   5篇
  1973年   7篇
排序方式: 共有871条查询结果,搜索用时 15 毫秒
631.
Although France does not have a specific law dealing with shorelines management, various policies and policy instruments exist which enable a certain amount of control over the shoreline development process. This article discusses the various institutional arrangements in France as they relate to (1) problems of marine pollution, (2) the control of land‐based pressures for shoreline use and development, and (3) the creation of a new shorelands trust. The role of existing institutions, such as the six river basin agencies, is also discussed.  相似文献   
632.
Ten years after their implementation, the Alsatian prefectorial orders concerning emergency measures applied in case of exceedance of alert thresholds for ozone, nitrogen dioxide and, a pioneer feature of the French region, the PM, required an update. In 2007, the ASPA was mandated by the Bas-Rhin and the Haut-Rhin Prefectures to define and assess new emergency measures. The Alsatian emissions inventory first identified the activity sectors which would be potentially targeted by the measures, and then calculated the associated emission reductions. In association with the transportation agency, 3 scenarios were retained for concentration simulation: limitation of the speed limit at 70 km/h on freeways and major urban roads, alternated traffic in towns, and a combination of both measures. Then, an integrated modelling chain, including the CHIMERE and ADMS-Urban models, was allowed to simulate, for the three scenarios, the variation of ozone during the heat wave of 2003, and the variation of nitrogen dioxide and the PM during wintry episodes, for the cities of Strasbourg, Colmar and Mulhouse. Finally, the urban cartographies of the impact of the scenarios on the concentrations were crossed with the georeferenced residence data to estimate the population affected by threshold exceedances in each case. A weak impact was obtained for speed limit reduction, while it was significant for the alternated traffic; for the PM, the concentrations fell about 10 μg/m3 and the population exposed to threshold exceedance fell by a factor 4; for nitrogen dioxide, the concentrations were lowered by several dozen μg/m3 and the exposed population fell by a factor 2; whereas for ozone, we noted a slight increase in the concentration and the exposed population. At the same time as the implementation of the newly developed emergency measures, the evolution of knowledge and modelling shows that it would be technically justified to differentiate the measures, pollutant by pollutant.  相似文献   
633.
IntroductionTheworkonthermochemica1treatInntswithasadditiOnswasshmulatOdbybasicresearchinl982inChinall].SignilicantProgfesshasbeenmadedurillthepastdecadeinthefield.TheresultsshowthatontheonehandtheREe1emedscanacceleratethedecomPositionofthemediumandactiVatethecarburizedschceofsteelsandthusenhancethecoefficientofcarbontransfer(D").OntheotherhandatelevatedtemPeratUreREelements,althoughtheyhavemuchbiggeratOmicsizesthaniron,couddiffuseintobasedalloystOasignilicamdepth.MoreifllPortantiXtheP…  相似文献   
634.
CONSTRUCTINGPOLYCOMPETITORCDNASFORQUANTITATIVEPCRS.L.Reiner,Zheng,Shichun;D.B.Corry,R.M.Locksley(DepartmentofMedicineandMicro...  相似文献   
635.
Two hydrobiological transects across the East Greenland Shelf and the open waters of Fram Strait in summer were chosen to illustrate the distribution and production of phyto- and zooplankton in relation to water masses and ice cover. The parameters used were temperature and salinity, inorganic nutrients, chlorophyll a, primary production, phytoplankton species composition, abundance of the dominant herbivorous copepods Calanus finmarchicus, C. glacialis, C. hyperboreus, Metridia longa and egg production of C. finmarchicus and C. glacialis. Grazing impact of copepodites and adults of these four species was modelled for each station by using egg production rates as an index of growth. Seasonal development of plankton communities was closely associated with the extent of the ice cover, hydrographic conditions and the water masses typical of the different hydrographic domains. Four regions were identified from their biological activities and physical environment: The Northeast Water polynya on the East Greenland Shelf, with a springbloom of diatoms and active reproduction of herbivorous copepods. The pack ice region, dominated by small flagellates and negligible grazing activities. The marginal ice zone, with high variability and strong gradients of autotroph production related to eddies and ice tongues, an active microbial loop and low egg production. The open water, with high station-to-station variability of most of the parameters, probably related to hydrographic mesoscale activities. Here, Phaeocystis pouchetii was a prominent species in the phytoplankton communities. Its presence may at least partly be responsible for the generally low egg production in the open waters. Grazing impact on primary production was always small, due to low zooplankton biomass in the polynya and due to low ingestion in the remaining regions.  相似文献   
636.
More than 40% of the registered fatigue cracks in ship structures are observed to occur in the side shell, more specifically in the connections of longitudinals to transverse web frames. The fatigue damage is caused partly by vertical and horizontal wave-induced hull bending and partly by outside water pressure on the side shell. Due to the non-linear nature of the outside water pressure the fatigue damage of the combined stress cannot be solved via a traditional frequency domain analysis. This paper solves the combination problem by application of a realistic wave model. The proposed model is used to analyze a segregated ballast tanker, and the results are compared to previously registered fatigue cracks. The analysis shows that it is very important to include the water pressure in the fatigue analysis, as this accounts for the majority of the expected fatigue damage. The analysis is performed for both I-shaped and L-shaped stiffeners.  相似文献   
637.
Spray sizing that records fluorescent emission and scattered light has been widely applied to spray diagnostics over the last two decades. Different experimental strategies have been developed, but comparing the different solutions offered has remained of interest to experimentalists. In this work, a comparison of two fluorescence strategies for measuring droplet size in the liquid phase of a last-generation DI diesel spray is conducted. The natural fluorescent emission of a commercial diesel fuel and the fluorescence emitted by a tracer (Rhodamine B) are compared using theoretical and experimental approaches. The LIF/Mie ratio commonly called Planar Droplet Sizing (PDS) technique is applied in two different ways to elucidate the possible advantages of using a fluorescent dopant. The sprays were injected under non-evaporative conditions into a constant pressure vessel that simulates densities present at the moment of injection in currently used passenger car diesel engines. Characterization of the signal properties was performed by measuring the absorption coefficient, fluorescence emission spectrum, quantum yield and lifetime of both configurations. The scattered light and fluorescence intensities were calculated to verify the dependencies of the droplet surface and volume. When applying the two techniques to quantify droplet size in dense diesel sprays, the results show that signal weakness and lack of control over the properties of natural fluorescence produce distortion in the shape of the spray and cause measurements to be unreliable.  相似文献   
638.
Particle number measurement is a new approach to determine emission, which may be more accurate at very low emission levels than when using gravimetric measurements. An experimental study was performed to investigate the effect of fuel properties on the performance, combustion process, regulated gaseous emissions and particle number emissions of a diesel engine with an uncatalyzed diesel particulate filter (DPF). The effect of the filter on the particle size distribution was reported. The DPF number-based filtration efficiency in terms of number efficiency and fractional efficiency for petroleum diesel fuel and two alternative fuels, BTL and GTL, were analyzed. For nearly all test modes, the filter had a higher number efficiency for diesel than for BTL and GTL. The DPF fractional efficiency showed it was highly dependent on fuel type and varied widely at each size range. For diesel, the filter fractional efficiency was sufficiently high and behaved as predicted by filtration theory. For BTL and GTL, the fractional performance of the filter decreased when unexpectedly low efficiencies within the nuclei mode were exhibited. This research will be helpful in understanding DPF number-based filtration performance for alternative fuels and will provide information for the development of particulate emission control technology.  相似文献   
639.
A dynamical model is presented to analyse the stability of motion of the two-wheeled 'Fokker F.28-like' landing gear including tyres. The model is equally applicable to similar landing gears. The influence of the introduction of a specially designed torsional damper on the dynamical behaviour of the landing gear-tyre combination is investigated. Results of the stability calculations are supported by test results.  相似文献   
640.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号